Method of eliminating the inclusion of gas bubbles when filling of brake fluid into a hydraulic automotive vehicle brake system

ABSTRACT

The present invention relates to a method of bubble-free filling of brake fluid into a hydraulic automotive vehicle brake system, which is equipped with slip control and/or automatic braking intervention, the pump of which, under operating conditions, delivers brake fluid from the secondary circuit into the primary circuit according to the return delivery principle. In this arrangement, exclusively the primary circuit is bled and filled with undersaturated brake fluid.

TECHNICAL FIELD

The present invention relates to a method of eliminating the inclusionof gas bubbles when filling of brake fluid into a hydraulic automotivevehicle brake system.

BACKGROUND OF THE INVENTION

German patent application No. 43 37 133 discloses a hydraulic automotivevehicle brake system with wheel slip control which is equipped withadditional non-return valves between the primary and secondary circuitsto let the air in the secondary circuit escape to the primary circuitduring bleeding of the brake system. The additional arrangement ofnon-return valves necessitates special structural modifications to thebrake system which automatically increase manufacturing costs.

Generic EP patent 0 543 187 describes a method of repeated vacuumcharging of the primary circuit of an anti-lock automotive vehicle brakesystem after the brake fluid has been removed from the primary circuitfor the purpose of installation into an automotive vehicle. To this end,the primary circuit includes a filter element downstream of a dampingchamber which is adapted to the surface stress of the brake fluid sothat the brake fluid under vacuum cannot escape from the dampingchamber.

Further, European patent application No. 275 351 discloses anundersaturated brake fluid which has only a low solubility of gases andwhich exhibits a certain ability to take up the residual air in thebrake system under vacuum.

An object of the present invention is to simplify a hydraulic automotivevehicle brake system of the above-mentioned type as regards thebubble-free filling process to such an effect that the design necessaryfor the hydraulic automotive vehicle brake system is maintained, and theneed for additional structural modifications for the purpose of bleedingand bubble-free filling of the secondary circuit of the brake system iseliminated. Also, it is desirable to absorb the air disposed in thesecondary circuit in the brake fluid in a fashion as good as possible.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts the basic circuit structure of a hydraulic slipcontrolled vehicle brake system which operates according to the returndelivery principle.

FIG. 2 shows in a diagram the pressure variation within the brake systemduring the bleeding and filling process.

FIG. 3 depicts in a diagram the switching activities of the functionelements shown in FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Based on the basic design of a slip-controlled automotive vehicle brakesystem illustrated in FIG. 1, initially, reference is made to the knownfeatures of the system which, for wheel slip control, includes an inletvalve element 4 and an outlet valve element 3. A first channel portion 1which accommodates the inlet valve element 4 leads from a brakingpressure generator 5 to a wheel brake 6. A second channel portion 2which includes the outlet valve element 3 and a pressure accumulator 7leads from the wheel brake 6 to the suction side of a pump 8. The pump8, with its pressure side, is connected to the first channel portion 1by way of a pump valve 9. Another pump valve is arranged in the secondchannel portion 2 and performs the function of a pump suction valve. Thesecond channel portion 2 is normally isolated from the first channelportion 1 by the outlet valve element 3 which is closed in the basicposition. This area of the hydraulic system, which is normally isolatedby electrically and hydraulically operable valves, thus, forms theso-called secondary circuit to which also the low-pressure accumulator 7is connected and from which the pump 8 aspirates pressure fluid whenneeded in the braking operation. FIG. 1 shows the primary circuit, whichcorresponds to the channel portion 1, filled with brake fluid after ableeding process and filling process. To illustrate the fillingcondition, the corresponding channel portion 1 is shaded in black. Priorto filling the primary circuit, which is typically carried out by thevehicle manufacturer in a conventional fashion or by way of high-vacuumfilling technology, there is an inclusion of air in the secondarycircuit which is now absorbed by the hydraulic fluid in the primarycircuit without bubbles due to the method referred to in the following.

The method of bubble-free filling of the hydraulic automotive vehiclebrake system which will be disclosed hereinbelow preconditions ableeding process of the primary circuit wherein all electromagneticallydriveable valve elements can remain in their initial position. A vacuumleakage test should generally follow the bleeding process to detect andeliminate any possible leakages. Typically, the primary circuit is actedupon by high vacuum. Appropriately, the bleeding process and arecommended vacuum leakage test is succeeded by a repeated bleedingwhich is followed by a filling process of the primary circuit withdegased brake fluid. All electrically driveable valve elements againremain in their initial position. The filling process of the primarycircuit concerns a pressure filling which should then be followed by aphase of stabilization of the fluid pressure in the primary circuit.Subsequent to the filling process of the primary circuit is a fillingprocess of the secondary circuit with the brake fluid of the primarycircuit.

The individual steps of the above-mentioned provisions for the bleedingand filling of the system are represented in the diagram of FIG. 2. Theabscissa in the diagram shows the time variation and the ordinate showsthe pressure variation during the filling process of the brake system.The relatively steep pressure decline corresponding to the curve portion‘a’ is apparent at the beginning of the bleeding process, which extendsalmost vertically relative to the abscissa towards the end of the firstbleeding process and rises slightly at the end of the leakage test Buntil, due to renewed bleeding, the desired high vacuum is reached inthe portion C. When the bleeding process is completed, the subsequentbrake fluid filling operation along the portion D makes the pressure(characteristic curve d) in the primary circuit rise considerably. Thephase of stabilization (portion E) follows before the pressure in theportion F of the diagram finally sets to the atmospheric pressure afterthe filling device has been disconnected.

Details of the subsequent filling of the secondary brake circuit withbrake fluid under high pressure from the primary circuit which isfollowed by the bubble-free phase of absorption of the air displacedfrom the secondary circuit by the brake fluid disposed in the primarycircuit will be explained by making reference to the subsequent diagram3. The brake pedal (characteristic curve g) is applied during theabsorption process so that a hydraulic pressure of approximately 50 to70 bar prevails in the brake system. The absorption process isprogrammed in order to have air inclusions of the secondary circuitresolved without bubbles in the brake fluid in any case. Among others,the program control provides that the pump (characteristic curve h) ispermanently running during the absorption process and the inlet valveelements (characteristic curve i) and the outlet valve elements(characteristic curve j) are electromagnetically energized in adetermined pulse/pause ratio. FIG. 3 shows the characteristics of thesquarewave pulses. This quasi causes chopping and better resolution ofthe air volume in the brake fluid volume to the end of accommodating theair in the brake fluid in the sense of the absorption process. By usingbrake fluid which is undersaturated in terms of its air contents, thebrake fluid can resolve the air volume and accommodate it withoutbubbles in the absorption process. Further, it can be seen in thediagram of FIG. 3 that the pulse length of the electromagneticallyexcited outlet valve elements (characteristic curve j) is smaller thanthe pulse length of the electromagnetically excited inlet valve elements(characteristic curves i) plotted against the time of the absorptionprocess along the abscissa. In addition, it can be taken from thediagram that the pulse lengths of the electromagnetically excited inletand outlet valve elements are superimposed on one another. In thepresent embodiment, the opening pulse for the inlet valve elements ofthe rear axle follows with time delay in relation to the opening pulseof the inlet valve elements of the front axle. Because the opening pulselength of the inlet valve elements of both characteristic curves i havebeen chosen to be equally long in time, the inlet valve elements on therear axle will close, offset in time, automatically after each openinginterval. The opening pulses for the outlet valve elements correspondingto the characteristic curve j are superimposed on the opening pulses ofthe inlet valve elements and amount only to a fourth of the on-cycle ofthe inlet valves approximately.

In case the above-mentioned slip-controlled brake system is supplementedby an electrically operable, normally closed change-over valve for thepurpose of automatic brake intervention, the pulse patterns shown inFIG. 3 must be supplemented by the switching cycles of the change-overvalve. Care should be taken that the pulse/pause actuation of thechange-over valve follows in about a course of the characteristic curvej, however, offset in time within the respective closing phase of theoutlet valves.

Advantageously, the need for electrically activating the outlet valveelements during bleeding is eliminated by the method according to thepresent invention. Also, no additional prefilling measures andstructural modifications to the system are required because the aircontents in the secondary circuit is accommodated in a brake fluidundersaturated with air in a relatively simple, program-controlledabsorption process.

LIST OF REFERENCE NUMERALS

1 first channel portion

2 second channel portion

3 outlet valve

4 inlet valve

5 braking pressure generator

6 wheel brake

7 low-pressure accumulator

8 pump

9 pump valve

What is claimed is:
 1. Method of bubble-free filling of brake fluid intoa hydraulic automotive vehicle brake system which includes a brake pedalfor the application of a wheel brake that is equipped with an automaticbrake intervention system which includes a pump of which delivers brakefluid from a secondary circuit into a primary circuit according to areturn delivery principle under operating conditions, wherein at leastone bleeding process and filling process with brake fluid under highpressure takes place that is related exclusively to the primary circuit,and wherein said hydraulic automotive vehicle brake system includes aplurality electrically operable valve elements which remain in theirinitial position during the bleeding process, comprising the steps of:filling the primary circuit with degased undersaturated brake fluidafter completing the bleeding process, pressurizing the brake fluid inthe primary circuit sufficiently highly such that air inclusions in thearea of the secondary circuit are conveyed into the primary circuit byway of the pump as soon as one of said plurality of electricallyoperable valve elements which normally isolates the secondary circuitfrom the primary circuit switches to an open position, applying thebrake pedal, operating the pump continuously, and exciting the inlet andoutlet valve elements electromagnetically in a pulse/pause ratio whichis responsive to the resolution of the air volume in the brake fluidvolume until the air inclusions of the secondary circuit are taken up inthe brake fluid of the primary circuit and resolved without bubbles. 2.Method as claimed in 1, further including applying a high vacuum to theunfilled primary circuit of the brake system.
 3. Method as claimed inclaim 1, wherein one of said plurality of electrically operable valveelements is an outer valve and another of said plurality of electricallyoperable valve elements is an input valve, and wherein a pulse lengthdelivered to the outlet valve is shorter than a pulse length deliveredto the inlet valve for the duration of the bleeding process.
 4. Methodas claimed in claim 3, wherein the pulse lengths delivered to the inletand outlet valve elements are superposed on one another.
 5. Method asclaimed in claim 1, further including the steps of conducting a vacuumleakage test following the bleeding process.